Thermoplastic polymers can be classified as semi-crystalline (at least partly crystalline), highly crystalline, or wholly or mainly amorphous. Crystalline or semi-crystalline polymers typically have a distinct melting point, as measured by differential scanning calorimetry (DSC). Crystalline and semi-crystalline polymers are typically melt-processed into fibers, films or molded articles.
Controlling the nucleation and crystallization rates of the polymers during melt processing is important in determining the physical properties of the finished article. Various methods of controlling nucleation and crystallization have been attempted, usually incorporating nucleating agents (e.g., inactive (physical) or active (chemical)) or completely changing polymers. Physical nucleating agents are typically finely divided particles and have a melting and freezing temperature greater than that of the polymer in which the particles are utilized while the chemical nucleating agents are primarily combinations of acidic and basic compounds such as organic or inorganic compounds. Unfortunately, the particulate nucleating agents can also detrimentally affect the physical properties of the final product, for example by lowering the tensile strength of film made from the polymers.
Copending application Ser. No. 07/609,286, filed Nov. 5, 1990, has solved this particulate nucleation problem by blending linear polyethylene with other polyethylenes to cause the blend to have an increased onset of crystallization. While this process is effective, economic considerations can also affect the ability of polymers to be melt blended cost effectively.
Many methods of polymerizing polymers and forming polymer blends to do specific jobs are disclosed in the literature. For example, U.S. Pat. No. 4,937,299 (Ewen et al.) teaches the use of a homogeneous catalyst system comprising at least two different mono-, di- or tricyclopentadienyls and their derivatives of a Group 4b, 5b and 6b transition metal each having different reactivity ratios and aluminoxane. The catalyst system is described as being homogeneous in a solution polymerization system and that the soluble (i.e., homogeneous) catalyst can be converted to a heterogeneous catalyst system by depositing it on a catalyst support. Thus, U.S. Pat. No. 4,937,299 describes their homogeneous catalyst system as a soluble catalyst system.
International Patent Application Number PCT/US89/04259 (Stehling et al.) discloses linear ethylene interpolymer blends of interpolymers having narrow molecular weight distributions and narrow composition distributions. The components of the blends are said to have narrow molecular weight distributions (i.e., the ratio of the weight average molecular weight to the number average molecular weight is less than or equal to 3.0). The components are said to be prepared by using metallocene catalyst systems known to provide narrow composition distributions and narrow molecular weight distributions. The desirable molecular weight and composition distributions are said to be obtained by blending different components or by polymerization of the blend components in the same or multiple reactors.